NOK 5G21 Gateway Battery Backup

It seems that it is more efficient to use a 12VDC UPS, versus 120VAC UPS, then additional conversion loss from AC-DC adapter. The link to the APC UPS above shows as unavailable. Here's a link to something that seems to work, but doesn't include the battery.
https://www.ebay.com/itm/284452109699?_trkparms=amclksrc%3DITM%26aid%3D1110006%26algo%3DHOMESPLICE.SIM%26ao%3D1%26asc%3D234212%26meid%3D906647436be64da187f8fa0ef789f768%26pid%3D101113%26rk%3D3%26rkt%3D12%26sd%3D233512645971%26itm%3D284452109699%26pmt%3D0%26noa%3D1%26pg%3D2563228%26algv%3DDefaultOrganicWeb%26brand%3DBelkin&_trksid=p2563228.c101113.m2108

Here’s another version of the above on Amazon that includes the battery:

https://www.amazon.com/Belkin-BATTERY-BACKUP-REVISION-BU3DC001-12V/dp/B004RRFY5I/ref=sr_1_5?dchild=1&keywords=Belkin+Battery+Backup+Unit+Rev-B+BU3DC001-12V+Residential+Gateway&qid=1633212730&sr=8-5

Looks like it uses a 12V 7.5AH battery; should last for about 3 hours at a draw of 2A but others have reported between 1 and 2A. Probably 3A if it is charging the interal battery too.

I wonder, if ever, will TMO allow the internal battery be used for both WiFi and Cellular during a power outage.

NOTE: buyer beware, I'm not suggesting buying this, just as an option. I would be best to buy something new with at least a 2-year warranty.